Environmental standards have enforced the oil industry to reduce the sulfur content in gasoline to a maximum value of 50 ppm by 2010, and further, to a maximum sulfur content of 10 ppm. This has motivated the search of materials and processes to comply the environmental laws on clean fuels, for which we considered a variety of options that involve the following:
(1) The use of new and more efficient catalysts, with more resistance to deactivation caused by severe operating conditions in the refining plants, such as T>375° C. and P>54 Kg/cm2 in the hydrotreating processes HDT, according to the design limits of the plants (F. Luck, Bull. Soc. Chim. Belg., 100, 11-12, (1991) 781; A. Corma, A. E. Palomares, F. Rey, Appl. Catal. B, 4 (1994) 169; and J. A. Wang, L. F. Chen, R. Limas-Ballesteros, A. Montoya, J. M. Dominguez, J. Molec Catal A: Chemical, 194 (2003) 181-193).
(2) Less severe operation conditions, for example the use of flow space velocities (LHSV) lower than those currently used, as well as higher capacity reactors (2-3 times the current capacity) to increase the contact time (τ=m/F) of the reactants with the catalyst active sites.
(3) The adsorption of sulfur and nitrogen contaminants that are present in FCC gasoline, using selective adsorbents (S. Mikhail, T. Zaki, L. Khalil, Appl Catal A: General, Vol 227, 1-2, 265-278 (2002); and “Effect of nickel and vanadium on sulfur reduction of FCC naphtha”, Appl Catal A: General, Vol 192, 2 (2000) 299-305).
In all cases, knowledge of the composition of FCC gasoline is a fundamental point of departure, for developing and implementing the technologies as mentioned. Generally, the gasoil fractions are fed to the FCC process reactors, which may be variable mixtures of light and heavy gas oils, which have a sulfur content higher than 2,000 ppm and about 1,291 ppm of nitrogen. However, due to the cracking and recombination processes that occur in the FCC reactor, the sulfur content in the product generally contains about 1,500 ppm sulfur and 300 ppm nitrogen, when the feedstock exceeds these figures. Therefore, the composition of sulfur and nitrogen of the original molecules is modified, since a greater concentration of complex molecules, i.e., 1-,5-dimethyldibenzothiophene (1-,5-DMDBT); 4-,6-dimethyldibenzothiophene (4-,6-DMDBT); Carbazole, Quinoline, Indole, etc.; there is a predominance of thiophenes and benzothiophenes in the FCC products, amounting to about 1,500 ppm total sulfur and 300 ppm nitrogen in the FCC gasoline
Apart the technology here stated, alternative technologies have emerged, such as the transformation process Octgain Exxon-Mobil; Octgain process, which is an example of a method for the nonselective desulfurization-hydrogenation reactions. A distinctive feature of this process is that by careful hydrogenation the level of desulfurization and an isomerization reaction of paraffins increase, which is used to offset the reduction in octane due to olefin saturation. Thus, the octane number is maintained at a minimum of 85 to 88 RON.
Table No. 1 shows the comparative operating conditions between the two processes, where one notes that the percentage of sulfur and nitrogen removal is similar in the two processes, but the process of the present invention achieves such removal level using operating conditions less severe than octgain process,
TABLE NO. 1comparative operating conditionsOctgain Process vs Present Invention% Removal ofOperatingOperatingsulfurTemperature (° C.)Pressure (atm)Octgain Process90.340040Present Invention90251
Other alternative processes such as the use of catalysts for the purification of gas streams containing chlorine, fluorine, sulfur, nitrogen, mercury and silicon efficiently allow the removal of these components (i.e., sulfur and nitrogen), which are based upon the selective adsorption of sulfur and nitrogen molecules; these materials have a potential advantage further characterized by a low severity operation (eg., T=0-25° C., P=1 atm.) while retaining the octane number of FCC gasoline.
From a molecular standpoint, typical fractions of diesel oil have properties that are similar to complex fluids, i.e., API density<22° API, Viscosity>1,000 cP, etc., with a typical high molecular weight composition of polyaromatic hydrocarbon molecules, i.e., MW>400, together with some associated heteroatoms, such as sulfur (S) and nitrogen (N), for example, carbazole, dibenzothiophene (DBT) and dimethyldibenzothiophene (DMDBT). The composition of these products is determined usually by separation and analysis techniques, for example short-path distillation (SPDA) with Simulated Distillation Chromatography (GCSD) and fractions separation (SARA) with high analytical performance (TLC-FID). Also there are other useful chromatographic techniques that are based on ion exchange or molecular diameters exclusion, in combination with some spectroscopic techniques like NMR, Tandem MS, MS, FI, EPR, etc.
Some heavy oils and diesel feeding the FCC process have a high proportion of polyaromatic compounds (asphaltenes), which are composed by 3 to 10 aromatic rings and a side chain having 3 to 15 carbon atoms, as well as some acid and basic polar groups with external heteroatoms such as sulfur, nitrogen, oxygen and metals (Ni and V).
In contrast, the products from the FCC process are both linear paraffins and branched olefins, which have a typical molecular weight within the range of gasolines (C8-C12+), as well as primary aromatic molecules with short alkyl chains (C2-C3) and naphthenic compounds.
Instead, heteroatoms (S and N) are associated to simple molecules such as thiophene and methylbenzothiophenes, which result mainly from the cracking and recombination processes in the FCC reactor. From a fundamental perspective, sulfured molecules that are present in FCC gasoline are characterized by their basic Lewis type character, namely a tendency to donate electrons, from the available electron pair of the sulfur atom, but the stability of the thiophenic ring causes a resistance towards the desulfurization reaction.
During the development of the present invention the primary criteria for designing effective adsorbents for contaminants removal were operating conditions such as T<30° C. and P=1 atm, in order to preserve the desirable properties of the FCC products, that is the octane number and olefin content in the liquid fraction (RON>85, Olefins>10%). Moreover, mesoporous silicates with pore diameters within the interval 2 nm<Dpore<50 nm have been the subject of great interest in the last 15 years, because they have pore diameters of 3 to 50 times larger than the pore diameter of other materials such as zeolites (0.3 nm<DPoro<1 nm). These features are very important as they allow the diffusion of more complex molecules such as those typically found in heavy crudes. However, the structural stability of these materials, for example MCM-41, MCM-48, MCM-50, MSU, SBA, etc., is a major limitation for some applications involving high temperatures and wet environments, i.e., T>400° C., RH>60%). Alternatively, the moderated conditions that are used in low severity processes, i.e., P=1 atm and T<100° C. with relative humidity (RH) of less than 50%, can be attractive for the application of these materials. Previous studies have reported the preparation of mesoporous materials from the cooperative interaction of molecules of ionic and non-ionic surfactants such as CTAB (Cetyl trimethyl ammonium bromide), Pluronic (Ethylene-propylene polyoxides) with chemical species in solution. Recently, J. M. Dominguez et al., reported the synthesis of porous materials with novel structures (Micr. Mesop Mater, 66 (2003) 341-348), consisting of silicate spheres with a radial distribution of interconnected pores. These materials have high specific areas, i.e., about 1,000 m2/g. Also, other porous silicate structures consisted of particles with elliptical geometry and a set of channels that run perpendicular to the major axis, with thick walls between pores (i.e., thickness of the order of 3 nm), which are more resistant than conventional materials based on MCM-41 (i.e., E. Terrés et al., U.S. Patent 2004/0052714 A1).
Therefore, the knowledge obtained in the synthesis of nanostructured materials was used in the present development as a conceptual basis for their application in the modification of surface properties such as acidity (Lewis). The results covered by the present invention were obtained from a comprehensive experimental investigation, by considering a variety of chemical compounds in order to obtain potential adsorbents, which might be comparable or superior to some commercially available materials, with regard to its properties for removal of sulfur and nitrogen compounds from FCC gasoline.